Technics RS-AZ6 trouble

[geminni]

I have this deck since the first production year, new. I did not used it so much. Couple of years i did not used it at all.
The thing is this:
I put in the tape, rewind it to beginning and play it. It plays few seconds ok then starts to fade out hi frequencies and continue to sound like the tape is puling out but it is not, it's wobbling sound
At that point i pres stop and then play, and the sound is again ok and after few sounds it starts to do the same thing again. But, if i fast forward it couple of minutes, after that it is ok the whole side of tape.

What can be the problem?